1. Field of the Invention
This invention relates to the treatment of wood and wood products to kill plant pests and preserve the fresh-cut characteristics of green wood. In particular, it relates to subjecting raw or green wood and wood products to heat for a sufficient time to destroy any plant pests that may have infested the wood or wood products, while preventing cracking, checking or other deterioration of the wood or wood products.
2. Discussion of the Related Art
Already, the United States has become the world's leading importer of wood and wood products. In 1990, the United States imported the equivalent of 34.4 million cubic meters of logs, lumber and other wood products.
The United States' demand for imported wood and wood products can only be expected to increase. Domestic logging companies are facing increasing challenges from conservation groups. Conservationists oppose many tree harvesting practices, especially clear cutting. In addition, concern over habitats for wildlife and preservation of stream beds for fish has impacted the supply of trees for harvest. Unanswered questions about replacement of old growth/diversified forests and the costs of logs and logging have changed the industry's economics greatly. Accordingly, domestic commercial forest lands are projected to decrease by about 4 percent over the next 50 years. The resulting shortage will need to be met in part by importing wood and wood products.
Potential supplies for additional wood and wood products are located in such far away places as the former Soviet Union, New Zealand, Chile and Brazil. An obstacle to importing green wood and wood products from such distant locations is the danger that the foreign products could introduce and spread exotic plant pests throughout the United States.
At the same time, the United States has become the world's largest exporter of wood and wood products. However, there are also a number of plant pests native to the United States, such as nematodes occasionally found in southern pines, that are not found in overseas forests. These pests create an obstacle to exporting still more timber grown in the United States to overseas markets.
A number of methods are known for destroying plant pests. Unfortunately, they all suffer from drawbacks, especially when considered in the context of treating wood and wood products to be transported overseas. For example, it is known that heat-treating wood and wood products, typically in a kiln with an attendant reduction in the moisture content of the material, is an effective method for killing plant pests. Such heat-treatment processes require bringing the center of the material to a certain minimum temperature for a certain minimum period of time so as to dry the material without causing any cellular or structural degradation.
Therefore, kiln-drying processes have been employed that are best suited to carefully and precisely treat relatively small amounts of material. They are typically used for treating wood lots of up to about 350 cubic meters or equivalent volumes of wood products having a relatively thin cross-section such as sawn lumber having a cross-sectional dimension of about 2".times.10". Rarely is lumber having a cross-sectional dimension in excess of 6".times.10" kiln dried.
Furthermore, the capital cost of a suitable kiln is great, running from at least many hundreds of thousands of dollars up to millions of dollars. And it is often the case, that far away countries, rich in raw materials such as timber, do not have the facilities to saw mill logs into lumber, let alone kilns or other costly treatment facilities. It is a further drawback of kilns and other treatment facilities, that once constructed they are fixed in location and point of service. Consequently, a large number of such facilities would be required to treat all the lumber found in disparate locations spread across the globe.
Even when wood and wood products are treated overseas, there remains a risk of reinfestation by plant pests. Unless the wood and wood products' environment is carefully monitored and controlled, reinfestation can occur before the materials are loaded aboard the ship, or even after they are loaded, if the ship contains infested cargo that has not been treated.
One approach to the problem is fumigating wood and wood products once a ship carrying a load of wood or wood products has completed its journey. This is a customary practice of eliminating pests in the both the United States and other countries, such as Japan. Fumigation effectively controls plant pests that may be associated with the surface and subsurface of debarked logs and other wood products. Fumigation may not be effective in killing other plant pests that bore deep into the wood or in killing microscopic pests that live in the wood's cells. Another significant drawback of fumigation is that it has been known to pose a health risk to people and the environment.
Additional disadvantages of fumigation include the expense of the fumigant. And, because of the potential health hazard of the fumigant, most often methyl bromide, fumigation can only be carried out under carefully controlled circumstances, usually, once the ship has entered port and the crew has been safely removed. This further adds to the environmental risk, time and expense involved in importing wood and wood products which are treated in this manner.
It is also known that boiling or steam-treating logs, so they may be softened and readily peeled in the initial steps of making veneer or plywood, can produce a side benefit of pest treatment. This land-based process, however, is not used for treating large quantities of logs or logs typically longer than about 10 feet. Nor is such boiling or steam-treating used for wood or wood products that are made into lumber or not immediately thereafter turned into veneer or plywood. Furthermore, this process takes place only after importation of the logs and usually at a location in or near domestic forests, where plant pest risks pose the greatest threat to domestic trees.
Another obstacle to importing green wood and wood products, especially logs, lumber, wood chips or wood strands from across the seas is the condition of the wood or wood product when it arrives at its destination. A freshly cut log has a moisture content of about 50%. As a general rule, because of the evaporation of surface and internal moisture, the longer the period of time since the tree has been cut, the drier the wood becomes. The increasing dryness of a log is a drawback in such subsequent manufacturing processes as the manufacture of lumber or veneer. If care is not taken during the period after felling the tree and continuing up through its shipping, moisture variations cause degrees of wood degradation such as cracks and checks. Moreover, incipient rot can form. These phenomena all make portions of the wood unusable in or less valuable for subsequent fabrication, such as fabrication into lumber or veneer or oriented strand board. Similarly, if care is not taken with wood chips or wood strands, there can be a significant loss of fiber, which can destroy or greatly reduce the value of the wood.
Accordingly, there has existed a definite need for a safe, effective and inexpensive method for eliminating significant plant pest risks from green wood and wood products, including large volumes of green logs, sawn lumber, wood chips and wood strands transported overseas. There has also existed a need for a method which minimizes the risk of reinfestation of plant pests after the initial treatment. There has existed a still further need for a method for maintaining the fresh-cut characteristics of wood and wood products delivered from overseas by reducing the incidence of cracks, checks and incipient rot or, in the case of wood chips or wood strands, by minimizing fiber loss. The present invention satisfies these and other needs and provides further related advantages.